SPAWNING, AGE DETERMINATION, LONGEVITY, AND MORTALITY OF THE SILVER SEATROUT, CYNOSCION NOTHUS, IN THE GULF OF MEXICO 1,2

Cynoscion nothus females from the Gulf of Mexico off Texas matured at 140-170 mm SL as they approached age I. Spawning occurred from early May through late October but primarily in two periods, May and August-September. Greatest spawningoccurred in the August-September period when two distinct spawned groups (intrayear class cohorts) were produced. The multiple-spawned group structure within a year class may be important to the population dynamics and stability of C. nothus. This species reached 130-190 mm SL at age 1. Only one year class occurred or dominated in anyone month, and only two year classes were ever present at once. The largest specimen captured was 190 mm SL, and 99% were <160 mm. The maximum life span (t,J was only 1-1.5 years off Texas but might be 2 years in the northcentral gulf. The total annual mortality rate was best estimated at 99.83% and probably is no lower than 90% if the life span is as long as 2 years. Larger C. nothusalmost disappeared during winter suggesting an offshore movement for overwintering. The silver seatrout, Cynoscionnothus, smallest of four congeners found along the U.S. Atlantic coast, ranges from Chesapeake Bay to the Bay of Campeche (Hildebrand and Schroeder 1928; Hildebrand 1955). Cynoscionnothus is one of the more abundant fishes of the nearshore waters of the northern Gulf of Mexico (Hildebrand 1954; Moore et al. 1970; Gutherz et al. 1975). It is not considered an estuarine species (Ginsburg 1931; Hildebrand and Cable 1934), but small numbers of C. nothus have been taken in bays throughout the year (e.g., Gunter 1938, 1945; Swingle 1971; Dahlberg 1972). Despite its abundance, no published study has been directed at C. nothus in the gulf. Little is known about its life history, although Mahood (1974) described spawning and monthly size composition in Georgia waters. General notes on C. nothus appear in many faunal studies, includ­ ing Hildebrand and Cable (1934), Gunter (1938, 1945), Chittenden and McEachran (1976), and in a literature review by Guest and Gunter (1958).